Emit a warning when using unsafe public key url

When encrypting secrets we use a public key retrieved from zuul. If we
get this key from an unencrypted url a man in the middle attack could
replace this encryption key. To make the user aware of this we should
emit a warning when using untrusted key sources.

Change-Id: I7f26e93d863be710a558e15fa1d086b223f465bf
1 file changed